Devices for adding and dropping wavelength coded signals (light of a specific wavelength or wavelengths) are known in the art. Such devices employ optical fibers which are utilized predominantly in telecommunications in addition to local area networks, computer networks and the like. The optical fibers are capable of carrying large amounts of information and it is the purpose of such devices of the present invention to extract a selected amount of information from the fiber by segregating the information carried on different wavelength channels.
Devices of this type are comprised of a variety of components which together provide the desired segregation of wavelength coded signals. Integrated optical couplers and especially directional couplers have been developed to accomplish evanescent directional coupling. Optical signals are coupled from one planar waveguide to another. The signals in the second planar waveguide propagate in the same direction in which the signals travel in the first planar waveguide.
Diffraction gratings (e.g. Bragg gratings) are used to isolate a narrow band of wavelengths. Such grating reflectors have made it possible to construct a device for use in adding or dropping a light signal at a predetermined center wavelength to or from a fiber optic transmission system without disturbing other signals at other wavelengths.
Wavelength division multiplexing systems are being deployed to greatly increase the band width capacity of existing optical fiber installations. Key components in these systems are the wavelength division multiplexers and demultiplexers that serve to combine and separate the individual wavelength signals at the two termini of the transmission system. These components include precision optical filters (e.g. Bragg gratings) that must be tailored specifically for each wavelength that is being transmitted. The number of wavelengths and their precise values vary from system to system and even within a system as a function of time as wavelength density increases.
The rapid growth of optical fiber-based telecommunications systems requires continual improvement in capacity of those systems to enable the management of increased bandwidth needs. There are several straightforward ways to increase the capacity of a system:
1. Install more optical fiber--this is the simplest approach but can be very expensive and time consuming; PA1 2. Increase the data rage of the transmitters on the end of the fiber--this is cheaper and quicker than installing new fiber, but at high data rates (&gt;5 Gigabits per sec), physical limitations of the optical fiber begin to be a problem, leading to unacceptably large dispersion of the optical pulse as it travels down the fiber; PA1 3. Transmit at low data rates at multiple wavelengths--once again, there is a cost savings over installing new fiber and now the primary challenge for the optical components is in being able to provide stable lasers at many wavelengths over the preferred range of 1530 to 1560 nm, and also providing precise filters that can segregate a desired wavelength.
Optical components as mentioned in Item No. 3 above may be deployed in wavelength division multiplexing (WDM) systems that carry 4, 8, 16, 32, 40 64, and 80 wavelengths of light simultaneously. A number of technologies have been used to solve the filter problem, among them fiber Bragg gratings (FBG) as disclosed in (No. 1), arrayed waveguide grating (AWG) routers as disclosed in (No. 2), and thin film dielectric filters as disclosed in No. 3. All of these approaches result in filter characteristics of varying quality, with the preferred filter characteristic being a transmission of 100% at the wavelength of choice +/- some range, and 0% transmission at all other wavelengths. In terms of dB units, filters are desired that provide greater than 20 dB and preferably greater than 30 dB discrimination between the preferred wavelength band and all other wavelengths.
With the exception of the AWG, all other filter approaches rely on a sequential use of discrete filter elements. This places a high demand on the quality of each filter element. Furthermore, since approaches such as FBG and thin film dielectric filters are by their nature fixed filters, and not tunable, each wavelength to be filtered requires its own, uniquely manufactured filter. As used herein the term "tunable" means that the filter element can be adjusted in a manner that will enable optical signals of different wavelengths to be segregated.
For example, a FBG suitable for the ITU wavelength 1547.72 nm will be unsuitable for the ITU wavelength 1550.92 nm, where the ITU wavelengths represent standard communications wavelengths that have been adopted by telecom system suppliers. This situation results in a considerable increase in the cost to manufacture the filters, and also increases cost of ownership because of time consuming labeling and inventorying of these devices. Thus, there is a need for a technology that provides for post manufacture adjustment of the filter wavelength, i.e. a tunable filter. By way of example, if 40 filter elements were needed, they could all be made identically and then adjusted, either at a factory or in the field, to filter the desired wavelength. This provides a greatly increased modularity to the WDM filter system, reducing cost of manufacture and ownership.
There are several tunable filter technologies that have been developed, chief among them acousto-optic tunable filter (AOTF) as disclosed in (No. 4) and Fabry-Perot tunable filter (FPTF). AOTF's, based on the acousto-optic effect present in ferroelectric materials such as lithium niobate, work by using an acoustic wave, stimulated by a radio-frequency power supply and transducer, to induce densification and rarefaction in an optical waveguide material. In practice, AOTF's usually work by changing the polarization of light that is at a wavelength that is matched to the acoustically induced grating. This light may then be separated from the other wavelength components present. AOTF's have the advantages of providing very rapid tuning (microseconds) and complete blanking of the filter (when the radio-frequency power is removed). However, it is very difficult to achieve the spectral characteristics desired for WDM by this approach, in terms of isolation between different wavelength channels, insertion loss at a given wavelength channel, and, in particular, polarization independence. FPTF's have been worked both in bulk embodiments as disclosed in (No. 5), and, more recently, via micromechanical approaches as disclosed in (No. 6). While FPTF's can achieve relatively good filter performance, they have the disadvantage of requiring a physical movement to achieve tuning, which reduces the overall reliability.
An ideal tunable filter technology would have both the solid state tuning of AOTF's coupled with the good filter performance of FPTF's.